1. Field of the Invention
The present invention relates to a light-emitting diode (LED) driving circuit and, more particularly, to an LED current-balance driving circuit for driving a plurality of LED strings.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional LED driving circuit for driving a plurality of LED strings. Referring to FIG. 1, an alternating-current (AC) power input 101A, such as a household power, is supplied to a commutation circuit 110 and converted to a direct-current (DC) power. The DC power is then supplied to a power factor correction (PFC) circuit 106 and converted to a high-voltage DC voltage VDC1 (approximately 80V˜400V). The high-voltage DC voltage VDC1 is supplied to an isolated DC-to-DC converter 107 and converted to a low-voltage DC voltage VDC2 (approximately 5V˜60V). The isolated DC-to-DC converter 107 divides the LED driving circuit in a primary side and a secondary side. Then, the low-voltage DC voltage VDC2 is supplied to a conventional LED current-balance driving circuit 108 located in the secondary side.
FIG. 2 is a circuit diagram showing the conventional LED current-balance driving circuit 108 in FIG. 1. Referring to FIG. 2, the low-voltage DC voltage VDC2 is supplied to a current control circuit 111A. The current control circuit 111A outputs a driving signal to control the conductive time of the transistor Q1 according to a current feedback signal generated by the resistor R1 coupled to the transistor Q1 so as to decide the time for the inductor L1 to charge the capacitor C1 through the diode D1. The output voltage VO across the capacitor C1 as well as the output power is thus controlled. Each LED string 105A,105B, . . . ,105M is series-coupled to a respective transistor QA,QB, . . . ,QM and a respective resistor RA,RB, . . . ,RM. The current control circuit 111A also outputs driving signals to control the transistors QA,QB, . . . ,QM according to the feedback signals from the resistors RA,RB, . . . ,RM so as to adjust the currents IOA,IOB, . . . ,IOM flowing through the LED strings to achieve the object of current balance.
However, the above mentioned LED current-balance driving circuit needs a plurality of transistors to control the currents flowing through the LED strings. When the number of the LED strings is increased, the power dissipation on the transistors will become significant.